(1) Field of the Invention
The present invention relates to an optical amplifier used in the field of optical communication, and in particular to an optical amplifier which amplifies wavelength division multiplexed (WDM) light which includes a plurality of optical signals arranged within a predetermined wavelength band.
(2) Description of the Related Art
Recently, progress in Internet technology has led to a rapid increase in the demand for information services, and in trunk optical transmission systems, further increases in capacity are required, and a flexible network structure is desired. The most effective transmission technology for meeting such demands on optical transmission systems is WDM transmission. WDM transmission is a method of transmitting a plurality of optical signals with different wavelengths over a single fiber optic cable by multiplexing the signals. At present, progress is being made in the commercialization of WDM optical transmission systems mainly in North America, for example.
A key component in the realization of a WDM transmission system is the rare earth doped optical fiber amplifier. A typical example of such a rare earth doped optical fiber amplifier is an EDFA (Erbium-Doped Fiber Amplifier) which uses optical fiber doped with erbium (Er3+) in the core (Erbium-Doped Fiber: EDF) as the amplification medium. An EDFA is an optical amplifier capable of amplifying WDM light collectively using the wide gain band obtained by the induced emission of erbium pumped by a pumping light supplied to the amplification medium. The first EDFAs to be commercialized were those corresponding to a wavelength band of approximately 1530 to 1560 nm (C-band) where the amplification efficiency is highest, and at present, EDFAs corresponding to the wavelength band of approximately 1570 to 1610 nm (L-band), which has the next highest amplification efficiency after C-band, are also being commercialized.
EDFAs as described above are used in optical repeaters and the like in WDM optical transmission systems. The most widely used optical repeaters and the like are multistage devices where a plurality of EDFs are connected in a cascade manner, which realize relatively flat gain wavelength characteristics (see Japanese Unexamined Patent Publication No. 7-202306, for example).
The power level of the WDM light which enters such an optical repeater varies according to such factors as the degree of loss in the transmission path which precedes the optical repeater. When this variation in the level of optical input to the optical repeater results in the total gain of the multistage EDFs deviating from a constant value, a problem occurs in that the flatness of the gain wavelength characteristics is no longer maintained, which causes tilt, and results in deterioration in the quality of such characteristics as the transmission distance and transmission band of the WDM optical transmission system.
FIG. 10 is a diagram showing the results of measuring the variation in the gain wavelength characteristics according to the optical input level of the optical repeater. Here, WDM light composed of 8 channels of signal light spaced evenly within a signal wavelength band of 1575 nm to 1610 nm, for example, was input into an optical repeater, and the input power of the WDM light was changed within a range from −14 dBm/ch to −6 dBm/ch (dynamic range 8 dB), while measuring the amount of deviation from the gain wavelength characteristics observed at an optical input level of −10 dBm/ch. The vertical axis in FIG. 10 indicates the amount of deviation (dB) of the gain wavelength characteristics, and the horizontal axis indicates the wavelength (nm).
From FIG. 10 it is apparent that when the EDFA is controlled such that substantially flat gain wavelength characteristics are obtained when the input level of the WDM light is −10 dBm/ch, when the input level changes from −14 dBm/ch towards −6 dBm/ch, the tilt of the gain wavelength characteristics changes from negative (an incline down and to the right in the diagram) to positive (an incline up and to the right in the diagram). The occurrence of such gain tilt causes the transmission quality to deteriorate.
In order to prevent this deterioration in transmission quality resulting from gain tilt caused by variation in the input level as described above, conventional technology is known whereby deterioration in the flatness of the gain wavelength characteristics is compensated for by providing a variable optical attenuator (VOA) between stages in a two stage EDF, for example, and keeping the gain of each EDF constant by adjusting the pumping light level while monitoring the input/output levels of the first stage and the second stage EDFs (see Japanese Unexamined Patent Publication No. 8-248455, for example).
FIG. 11 is a diagram showing an example of a construction of a conventional optical amplifier which can maintain flat gain wavelength characteristics over a wide input dynamic range. In this conventional optical amplifier, the power of WDM light Ls input to and output from a first stage EDF 111 is monitored by optical branching couplers 114 and 116 and photodetectors (PD) 115 and 117, and the power of the pumping light Lp1 supplied to the EDF 111 from a pumping light source (LD) 112 via a multiplexer 113 is controlled by an AGC circuit 118 so that the gain calculated based on the results of this monitoring equals a predetermined target gain A. Furthermore, on a second stage EDF 121 side also, in the same manner, the power of pumping light Lp2 is controlled by an AGC circuit 128 so that the gain calculated based on the results of monitoring by photodetectors (PD) 125 and 127 equals a predetermined target gain B. In addition, the power of the WDM light Ls output from the second stage EDF 121 is determined based on the results of monitoring by the photodetector 127, and the amount of optical attenuation of a VOA 131 is controlled by an ALC circuit 132 so that the optical output power equals a predetermined target value. After the amount of optical attenuation of the VOA 131 is determined, the ALC control loop is opened and this amount of optical attenuation is maintained. Because when this controlled state is entered, the gain in the EDFs 111 and 121 is controlled by the AGC circuits 118 and 128 so as to remain constant even when there is a change in the number of wavelengths of the optical signals within the WDM light Ls, an optical amplifier with flat gain wavelength characteristics can be realized.
FIG. 12 shows an example of an energy level diagram according to input level, for such a conventional optical amplifier. It is apparent from FIG. 12 that the EDF gain A and B for each stage is controlled so as to remain constant regardless of variation in the input level, and that the amount of optical attenuation of the VOA 131 increases or decreases according to the input level.
However, in metro network systems and the like, demand for which has continued to build in recent years, because the number of wavelengths in the WDM light varies freely over a wide range during transmission, it is desirable that the pumping light of the optical amplifier be controlled at a high speed to continually maintain constant gain even when faced with dynamic and high speed variation in the number of wavelengths.
However, with this example of a conventional construction shown in FIG. 11, when the number of wavelengths in the WDM light is increased or decreased, the gain is kept constant by repeating a control loop wherein the pumping light power is adjusted according to the amount of gain deviation relative to the target gain A or B in each EDF stage at the current point in time. But with such a control system, a problem occurs in that limitations imposed by the speed limit of the control circuit cause large fluctuations in gain whenever the number of wavelengths in the WDM light is increased or decreased.
A construction has been proposed for a conventional optical amplifier where high speed control is performed as a means to cope with this problem. For example, as shown in FIG. 13, a wavelength number calculation circuit 243 which recognizes changes in the number of wavelengths in the WDM light based on input monitoring results obtained by an optical branching coupler 241 and a photodetector (PD) 242 provided upstream from a first stage EDF 211 is provided, power settings are obtained in advance for pumping lights Lp1 and Lp2 at which settings the overall gain of the optical amplifier relative to the number of wavelengths remains constant (A+B=constant) and are stored in a memory circuit 244, and the power of the pumping lights Lp1 and Lp2 supplied to the EDFs 211 and 221 is controlled by a pumping light power control circuit 245 according to the number of wavelengths as recognized by the wavelength number calculation circuit 243 and the corresponding information in the memory circuit 244 (see for example Reference Document 1: Cechan Tian et al., J. Lightwave Tech., vol.21, no.8, pp1728–1734, 2003. and Reference Document 2: Cechan Tian and Susumu Kinoshita, “Novel solution for transient control of WDM amplifiers using the combination of electrical feedforward and feedback,” CLEO2002, CW02, Long Beach, 2002). With such a conventional optical amplifier, because any change in the number of wavelengths is recognized quickly by the input monitor, and the number of AGC loops reduces from two to one, the gain of the optical amplifier can be controlled at high speed so as to remain constant even when increasing or decreasing the number of wavelengths.
However, with the conventional optical amplifier as shown in FIG. 13, if the number of wavelengths in the WDM light changes, even if the overall gain of the optical amplifiers can be kept constant, the gain distribution (energy level diagram) for each stage changes freely depending on the settings used for the power of the first stage pumping light Lp1 and the power of the second state pumping light Lp2. Therefore the following problems occur.
For example, as shown in FIG. 14, when the number of wavelengths is a low number (for example 1 channel) as opposed to when the number of wavelengths is a maximum (for example 88 channels), in the case where the gain A of the first stage EDF 211 is large and the gain B of the second stage EDF 221 is small, the input level of the second state EDF 221 is higher when the number of wavelengths is low. Therefore the total ASE optical power generated in the optical amplifier will be smaller. Regarding the fluctuations which occur in the ASE optical power when the number of wavelengths changes, these remains unstable momentarily due to the response speed of the EDF. Therefore, as shown in FIG. 15 for example, even assuming an ideal situation where the speed of the control circuit is sufficiently fast, and where immediately after reducing the number of wavelengths there is no fluctuation in the total optical power, that is the sum of the residual channel power and the ASE power, a phenomenon occurs whereby the ASE optical power fluctuates slowly in a delayed manner before reaching a stable state. As a result, as shown in FIG. 15, there occurs a phenomenon whereby the residual channel power fluctuates towards the minus side when the number of wavelengths is reduced. This fluctuation of the residual channel power also occurs to an equivalent degree in repeaters positioned on the downstream side. Therefore, the fluctuation accumulates for each relay amplifier present, and exceeds the tolerance for fluctuation able to be received by the receiver. FIG. 16 shows the data from experiments relating to the transitional fluctuation of the residual channel power mentioned above. After a reduction in the number of wavelengths in the input light is completed (corresponding to the dashed line in the diagram), 3 to 4 ms is required before the residual channel power stabilizes to the desired constant level, and this delay is related to the response speed of the ASE light.
In order to avoid such gain fluctuation phenomena, it is necessary to ensure that the ASE optical power does not change regardless of the number of wavelengths, that is ensure that the gain in the EDFs 211 and 221 does not change regardless of the number of wavelengths. A way of realizing this is to obtain in advance power distribution settings for each pumping light Lp1 and Lp2 at which the gain for both EDF 211 and 221 stages remains constant (A=constant, B=constant) with respect to changes in the number of wavelengths, as shown in the energy level diagram in FIG. 17, for example, and store this information in the storage circuit 244.
However, in an EDFA corresponding to the L band, for example, it is known that in order to ensure that the gain of the EDF 211 and 221 stages is constant with respect to changes in the number of wavelengths, the power of each pumping light Lp1 and Lp2 must be controlled in a complicated manner following a nonlinear relationship as shown in FIG. 18, for example, in accordance with the number of wavelengths. Specifically, from the characteristics example in FIG. 18 it is apparent that in a range where the number of wavelengths is less than a given constant, that is, the input power is less than a given constant, the required pumping light power tends towards the increase. This indicates a phenomenon where in the L band EDFA, the amplification efficiency deteriorates when used in a region where the input power is less than a given constant.
The following is a description of the theory of this deterioration in amplification efficiency, with reference to FIG. 19.
In a forward pumping type L-band EDFA as shown in the upper part of FIG. 19, if the input power becomes low, the population inversion at the part near the signal entry end of the EDF becomes very high, as shown in FIG. 19 (A). As a result, there is a large amount of C-band ASE light generated in this part, as shown in FIG. 19 (B). In addition, while the ASE light of the ASE light within this C-band, which is traveling in the opposite direction to the signal light propagates towards the signal entry end, then as shown in FIG. 19 (C) the pumping light is wasted in amplification of this ASE light traveling in the opposite direction, which results in deterioration of the amplification efficiency.
Accordingly, there is a problem in that the complicated control of the pumping light power in an L band EDFA as shown in FIG. 18 leads to a reduction in the control speed of the electric circuit and an increase in the cost of the electric circuit.